Online reading assignment: circuit elements

Physics 205B, spring semester 2014
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on circuit elements.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Current is the amount of charge flowing through a conductor or resistor measured in units of amperes or coulombs/second. Current flowing is the same thing as electrons moving the opposite direction. Fresh batteries are close to being ideal. Ones that have been used and worn down will have a voltage less than the electromotive force."

"I understand that an object with high resistance will have less current flowing through it than an object with low resistance when they are connected to the same voltage source. I also understand that current is the direction of flow of positive charge and therefore is moving in the opposite direction as electrons."

"I understand how batteries become 'dead.' Batteries are basically little chambers for a bunch of chemical reactions that release energy in the form of electric potential. Once the reactants of the chemical equation are used up, the reactions can't occur and so no more energy can be released."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I'm still getting confused about when to use what equation. I often struggle with regards to knowing what each symbol stands for and how you recognize them in problems."

The emf and how it is related to the potential difference in a battery was confusing. I'm not completely sure how those are related to an ideal battery."

"I don't really understand Ohm's law and how to apply it. I am also confused as to why when temperature increases, resistivity increases."

Identify the SI unit for these circuit element quantities.
(Only correct responses shown.)
Capacitance C: farads [70%]
Charge q: coulombs [78%]
Current I: amperes [81%]
Electric field E: newtons per coulomb [65%]
Electric potential energy UE: joules [60%]
Electric potential difference ΔV: volts [76%]
emf (electromotive force) ε: volts [57%]
Resistance R: ohms [87%]

Describe why batteries are typically stacked in series, positive terminal connected to the adjacent negative terminal of each neighboring battery.
"Batteries are typically stacked in series to increase the voltage and allow the batteries to power a higher variety of things."

"If they are stacked in series the energy can flow through each battery instead of just one."

"So the current will pass from battery to battery."

"I feel like it has to do with the fact that negative is attracted to positive, but I'm not sure beyond that."

"I'm not really sure why."

Explain the difference between an ohmic conductor and a non-ohmic conductor.
"Non-ohmic conductors do not obey Ohm's law and do not a have constant value of resistance. Whereas ohmic conductors follow Ohm's law, have a constant value of resistance and their graph is a straight line."

"Ohmic conductors have a linear relationship between current and voltage. A non-ohmic conductor has a resistance that varies based on how much current/voltage is going through it."

"I'm not sure."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I am not clear on the difference between resistance and resistivity. Except that resistance depends on size and shape while resistivity only depends on temperature." (Resistivity is a property of a given material, for example, at room temperature carbon has a resistivity of ρ = 3.5×10–5 Ω·m. When you shape this carbon into a cylinder of a certain diameter and length, you now have a resistor that you can run current through, and it will have a given value of resistance in ohms.)

"I don't really understand the way protons or electrons travel. How does that work?" (Protons (in atomic nuclei) don't travel in a conductor. Their outer electrons do. But since current is defined as the flow of positive charge, then the direction of current is opposite that of electron flow.)

"What would happen if batteries were not stacked in series with the positive end connected to the negative end?" (Their potentials would subtract from each other, instead of adding.)

"I'm a little confused at why the size of a battery doesn't dictate the emf, when a larger battery pumps more but does the same amount of work per unit charge?" (Different size batteries with the same emf, such as "AAA," "AA", "C" and "D" cells have the same chemical reactions inside, so the same energy per coulomb is released for each reaction. However, the amount of chemicals inside is different, so a "D" cell will last much longer than an "AAA" cell.)

"15,000 volts, two nails and a piece of (wet) plywood can make some bitchin' art. Where can I get my hands on a 15,000 volt potential source?" (Surplus neon sign transformers, like from eBay. Caveat emptor. Caveat utilitor.)

"I don't understand how a coil in a magnetic field can generate an electric potential." (Shh...wait until Chapter 20.)

No comments: